- non-expandable, non-contractable, freely communicating space, the pressure of fluid contents & brain itself, must be directly porportional to eachother in order to maintain a constant pressure
- if one of these pressures increases, another must decrease in order to compensate

- significant reduction in compliance
- any small addition of volume causes a great increase in ICP
- compensatory mechanisms fail: loss of autoregulation & pt will show s/s of increased ICP [HA, ALOC, or pupil responsiveness]